Conventionally known fine porous membranes for precise filtration used in the pharmaceutical industry, food industry, and electronics industry, include those made of cellulose esters, aliphatic polyamides, polyfluorocarbon, polysulfone, polypropylene, etc. as disclosed, e.g., in Japanese Patent Publication No. 40050/73 and Japanese Patent Application (OPI) Nos. 37842/83, 91732/83, and 154051/81 (the term "OPI" as used herein means "unexamined published Japanese patent application").
These conventional fine porous membranes are classified into two large groups, one of which is called a symmetric membrane in which the inside pores have a substantially unchanged diameter throughout the thickness direction so that the pores on both sides thereof have substantially the same diameter. The other is called an asymmetric membrane in which the pore diameter changes continuously or discontinuously in the thickness direction so that the pore diameter on one side thereof is different from that on the other side.
As described in Japanese Patent Application (OPI) No. 9801/83, the symmetric membrane has a large resistance, as a whole, to a liquid stream to be filtered. As a result, it has a small flow rate, i.e., the flow rate per unit area per unit time per unit differential pressure is small. In addition, it is easily clogged, has a short working life, and lacks anti-blocking properties.
On the other hand, the asymmetric membrane contains a so-called dense layer having small pores on one side of the membrane and relatively large pores on the other side as described in Japanese Patent Publication No. 6406/80 and Japanese Patent Application (OPI) No. 154051/81. Since the smallest of the particles to be filtered out can be substantially trapped by the dense layer, the entire thickness of the membrane can be effectively used as a filter medium. Therefore, if care is taken on use, it is possible to increase the flow rate or increase the life of the membrane. From this viewpoint, the asymmetric membrane has excellent performance as a fine porous filter membrane.
However, in spite of the extreme importance of the dense layer, because the conventional dense layer is present on the surface, it easily receives scratches, which leads to leakage of fine particles.
In order to overcome such a disadvantage, studies have been directed to a filter membrane having a dense layer, i.e., a layer having small pores, in the interior thereof. For example, Japanese Patent Application (OPI) No. 150402/83 proposed a discontinuous structure composed of two asymmetric membranes with each of the dense layers thereof being intimately contacted together. However, when such a filter system composed of two asymmetric membranes is placed in a cartridge in a fold, the filter area in the cartridge becomes small which reduces the flow rate as a module. For this reason, it has been keenly desired in the art to develop a filter medium composed of a single membrane containing a dense layer in the interior thereof.
Aiming at a solution of the above-described problems, a dry wet process, one of the processes for producing fine porous membranes, which comprises casting a polymer stock solution and allowing the cast film in air for a given period of time to cause microphase separation while controlling the pore size thus formed was extensively investigated. As a result, it was surprisingly discovered that a layer having the minimum pore size can be formed in a fine porous membrane in the interior along the direction perpendicular to its surface by properly controlling the evaporation loss of the solvent and the absorption of a non-solvent vapor as disclosed in Japanese Patent Application (OPI) No. 27006/87. This technique is unlike the method which includes sufficient evaporation of the solvent as disclosed, e.g., in Japanese Patent Application (OPI) No. 102416/80 or the method which includes dipping a film in a coagulating bath without substantial evaporation of the solvent as disclosed, e.g., in Japanese Patent Application (OPI) Nos. 8887/80 and 154051/81.